1. Technical Field
The subject matter described herein is concerned with wafers for fabricating semiconductor devices and methods of fabricating the same. In particular, the subject matter relates to a wafer in which a semiconductor layer is formed on an insulation film, and a method of fabricating the same.
2. Description of the Related Art
Active and passive elements constituting a semiconductor device are usually fabricated on a semiconductor wafer having a single crystalline orientation. High-performance semiconductor devices are needed to have high-frequency operations, lower power consumption, and to be free from malfunctions caused by undesired current paths. Those requirements for the semiconductor devices may be accomplished by adopting low-resistive interconnections, improving junction structures, providing high mobility of charges, reducing leakage currents, interrupting abnormal current paths, and so forth.
Transistors as typical kinds of the semiconductor devices are classified into NMOS transistors using electrons as carriers, and PMOS transistors using holes as carriers. It is known that electrons have a higher mobility when traveling normal to the (100) crystallographic orientation while the holes have a higher mobility when traveling normal to the (110) crystallographic orientation. The NMOS transistors are dominantly used in a general semiconductor device. The mobility of holes in a substrate with the (100) orientation is lower than that of electrons by two to four times. In order to compensate such a difference of mobility between electrons and holes, the PMOS transistor is designed to have a wider channel width, which increases an area of the semiconductor device. To the contrary, the mobility of holes is mostly twice that of electrons in the (110)-oriented substrate. Accordingly, the PMOS transistor is ideally fabricated in the (110)-oriented substrate, while the NMOS transistor is ideally fabricated in the (100)-oriented substrate.